The present invention relates to adhesive systems based on cationically curing compounds, and to their use.
In polymerizable dental compositions, use has to date been made predominantly of methacrylate monomers and acrylate monomers. Particular attentiveness is deserved by 2,2-bis[4-(2-hydroxy-3-methacryloyloxypropoxy)-phenyl]propane (bis-GMA), described by Bowen [U.S. Pat. No. 3,066,112]. Mixtures of this methacrylate with triethylene glycol dimethacrylate are still used even today as a standard formulation for dental plastic direct filling materials. Methacrylic derivatives of the diformylated bis(hydroxymethyl)tricyclo-[5.2.1.02,6]decane are also established as monomers for dental composites [W. Gruber et al., DE-A-27 14 538; W. Schmitt et al., DE-A-28 16 823; J. Reiners et al., EP-A-0 261 520]. A significant disadvantage of these dental compositions, however, is the high volume shrinkage that occurs as a result of the polymerization. This shrinkage may be minimized, for example, through the use of ring-opening monomers, such as the cationically curing epoxides.
Concerning cationically curable epoxide compositions for dental applications, little is known: U.S. Pat. No. 5,556,896 describes epoxide-containing compositions which are necessarily required to include spiroorthocarbonates as shrinkage-compensating monomers. Bowen describes a composition comprising quartz sand and an aliphatic diepoxide (bisphenol A diglycidyl ether) which in the cured state is said to have good stability in the oral environment [J. Dent. Res. 35, 1956, 360-379]. AT-A-204 687 describes epoxide dental compositions based on bisphenol A which are cured by means of Lewis acid catalysts. The documents DE-A-196 48 283, WO-96/13538 and WO-95/30402 likewise describe polymerizable dental compositions based on epoxides, and their use.
Although extensive experience exists with epoxides and cycloaliphatic epoxides (U.S. Pat. No. 2,716,123, U.S. Pat. No. 2,750,395, U.S. Pat. No. 2,863,881, U.S. Pat. No. 3,187,018), such monomers, and cationically polymerizable compositions formulated from them, have at no point in time been available commercially with the properties required for dental applications.
The reason for this is the fact that the curing of these cationically polymerizing compositions is inhibited by water and the hard tooth substance, such as in dentine, contains from about 11 to 16 percent by weight of water (G.-H. Schumacher et. al., Anatomie und Biochemie der Zxc3xa4hne, Gustav Fischer Verlag, 1990, 4th edition). Accordingly, it is easy to explain that, for example, formulations of dental filling materials on an epoxide basis are unable to exhibit inherent adhesion to dentine, and so the use of such materials was impossible.
In order to affix (meth)acrylate-based dental filling materialsxe2x80x94i.e., free-radically curing systemsxe2x80x94use is made of what are known as adhesive systems.
The quality of these adhesive systems is reflected in the following criteria:
complete adhesion to the hard tooth substance, without defects (xe2x80x9csealingxe2x80x9d);
complete adhesion to the filling material,
permanent bond.
In view of the circumstances set out above, the curing of cationically crosslinking systems on the hard tooth substance is unlikely. An adhesive system on a cationically crosslinking basis for compositions (including those) that cure cationically has never been available commercially on the market.
DE-A-197 43 564, it is true, describes radiation-curable adhesion promoterxe2x80x94so-called primerxe2x80x94compositions based on solvent-free, cationically and/or free-radically curable crosslinking systems; however, these systems are used only to coat water-free materials, examples being plastics, such as polyvinylidene chloride (PVDC) or silicone.
WO-98/47046 describes epoxide-based photopolymerizable mixtures comprising an epoxy resin, an iodonium salt, a transfer molecule sensitive in visible light, and an electron donor and claims their use as a dental adhesive system. However, it is found that with such mixtures it is impossible to obtain adhesion to cationically curing mixtures on the hard tooth substance (see comparative mixtures 1 to 3 of the present specification).
WO-99/34766 states that compositions with a high proportion of cationically curable groups adhere very poorly if at all to hard tooth tissue. To solve the problem, it is proposed to provide either a hybrid composition comprising constituents containing free-radically and cationically polymerizable groups or a composition which is substantially free from cationically polymerizable groups.
It is a primary object of the present invention to provide an alternative adhesive system which solves the abovementioned problems and can be adhered preferably to water-containing hard tissue, such as tooth.
In accordance with the invention, this object is achieved through the use of adhesive systems comprising at least one component i) capable of initiating a cationic polymerization and selected from inorganic acids having a pKA of less than 4.90, preferably less than 4.80, unsaturated carboxylic acids and their anhydrides or acid chlorides, unsaturated organic phosphoric acids and their esters, unsaturated organic phosphonic acids and their esters, unsaturated organic sulfonic acids and their esters, and at least one component ii) which is cationically polymerizable.
It has surprisingly been found that, when using cationically polymerizable adhesive systems with photopolymerization initiators such as are described in WO-98/47046 to affix cationically curing materials to water-containing hard tissue, adequate adhesion is oil unobtainable (see comparison mixtures 1 to 3 of the present specification).
This can be done only using adhesive systems which comprise the specific acids mentioned above and described hereinbelow.
Additionally, the photopolymerization inhibitors described in the aforementioned WO-98/47046, composed of an iodonium salt, a transfer molecule which is sensitive in visible light, and an electron donor, and also, for example, the cationic polymerization initiators described in DE-A-197 36 471 and DE-A-197 43 564, may be present.
It has further been found that a particularly advantageous method of affixing aforementioned materials to aforementioned hard tissue comprises using free-radically crosslinkable materials as acids and additionally conducting a free-radical crosslinking. In this way, the cationically crosslinkable groups are very largely retained for copolymerization with the filling material in the boundary layer.
A further adhesion-promoting effect of this reaction regime is the development of the xe2x80x9clubricating layerxe2x80x9d which is known to form in free-radically crosslinking systems. This layer comes about as a result of oxygen inhibition and provides for optimum mixing at the boundary between adhesive and filling system.